1. Field of the Invention
The present invention relates to electrically conducting films comprising at least one monomolecular layer of an organic charge transfer complex and the process for the production thereof.
2. Discussion of the Background
It is pointed out that organic charge transfer complexes are formed by the association of two molecules whereof one, A, acts as the electron acceptor and the other, D, acts as the electron donor.
This reaction can be represented in the following way: EQU mA+nD.fwdarw.A.sub.m.sup.-.rho. D.sub.n.sup.+.rho.
m and n representing the respective numbers of molecules and .rho. the charge transfer corresponding to the proportion of electron transferred. When m=n, there is a simple stoichiometry and when m.noteq.n, there is a complex stoichiometry.
When .rho..perspectiveto.0, the complex is a molecular complex in the neutral fundamental state.
When .rho.=1, it is a true ionic compound with one electric charge per D or A molecule and when .rho.&lt;1, there are fewer electric charges than molecules present. Thus, a mixed valence compound is obtained, which is a necessary condition for obtaining an organic conductor. A distinction can then be made between:
(1) the true charge transfer complexes in which there is a transfer of an electron from the donor to the acceptor, said single electrons then being on .pi. orbitals (.pi.-.pi. complexes) and
(2) radical ion salts in which only one of the ions is of a radical nature, the counter-ion being diamagnetic.
Complexes of this type are in particular described in Annales de Physique, 1976, Vol. I, No. 4-5, pp. 145-256 and in the Journal de Chimie-Physique, 1982, 79, No. 4.
These complexes produced either chemically or electrochemically crystallize in rows of aligned molecules, which gives a unidimensional conduction and these rows are surrounded throughout by identical parallel rows, whose immediate proximity induces by coupling a "transverse" conduction, i.e. a conduction perpendicular to the preferred direction. These electrically conducting or semiconducting complexes consequently have a dominant unidimensional character, mixed with the tridimensional type.
These complexes are obtained in the form of needles or crystals. However, it would be very interesting to have them in the form of homogeneous conducting films with in particular very thin conductor planes, which are insulated from one another and as would be the case in monomolecular layers.
Thus, for producing certain electronic components, structures of this type are of interest due to their high conduction anisotropy. However, hitherto, it has been impossible to obtain conducting films of this type having monomolecular layers of charge transfer complexes.
Thus, in order to be able to deposit monomolecular layers by the known Langmuir Blodgett process described in the Journal of Am. Chem. Soc. Vol. 57, 1935, pp. 1007 to 1010, it is firstly necessary for the charge transfer complex to be formed by an amphiphilic or amphipathic molecule, i.e. an organic molecule having a hydrophobic part, i.e. a part having a repulsion for polar liquids such as water, and a hydrophilic part, i.e. a part having an affinity for polar liquids, such as water. Thus, on introducing such amphiphilic molecules onto the surface of a liquid such as water, the molecules spread over the liquid surface and orient themselves in such a way that their hydrophilic part is immersed in the water, whereas their hydrophobic part, generally constituted by a hydrocarbon chain, tends to move away therefrom, in such a way that the axis of the chain becomes perpendicular to the surface of the water. If there are adequate cohesion forces between the molecules, the latter remain grouped and limit their spread to a continuous monomolecular film having essentially the thickness of one molecule, which corresponds to a Langmuir film. Such films can be manipulated on the surface of the water, then deposited on a solid support after having been compressed under an appropriate surface pressure. When a support is then introduced into the trough, the monomolecular layer on the surface of the water is deposited on said solid support. In the same way, on extracting a support which has been previously immersed in the trough, a monomolecular layer is deposited on said support and the orientation of the molecules with respect to the support is different as a function of whether deposition has taken place by introducing the support into the trough or by extracting the support from the trough. However, this process cannot be used on the presently known charge transfer complexes, because the latter are not formed from amphiphilic molecules.
However, by substituting one of the molecules of the complex by a hydrophobic hydrocarbon chain, it is possible to obtain amphiphilic complexes. However, the tests carried out up to the present in connection with the deposition of monomolecular layers of the thus substituted, amphiphilic, charge transfer complexes have not made it possible to directly obtain a conducting film. Thus, the charge transfer complexes only have electrically-conducting properties under certain conditions. It is in particular necessary for the molecules to be highly organised in a specific manner, in order to permit both the covering of the conducting orbitals and the partial charge transfer. This organization exists in complexes in the form of powders, but the need to dissolve these complexes in a solution for forming a Langmuir film destroys this organisation. The same applies when using processes involving the deposition of films by evaporation, sublimation, dissolving or any other process requiring a phase of separating the molecules from one another.